1. Field of the Invention
The present invention relates generally to welding material and the method of application therefor and, more particularly, to a welding material of ferrite-austenite two-phase stainless steel capable of producing a weld metal having a high resistance to corrosion, particularly in an environment containing nitric acid, and the method of application therefor.
2. Description of the Prior Art
In the past, ordinary austenite steels such as those specified in JIS (Japanese Industrial Standard) as SUS 304 and SUS 304L have been used as materials for structures which are used in a nitric acid environment having a comparatively low concentration of up to around 40 wt%. These materials exhibit sufficient resistance to corrosion under a nitric acid environment of such a low concentration. However, these materials exhibit a greater tendency of preferential corrosion of grain boundary, i.e., intergranular corrosion, as the concentration of the nitric acid in the environment is increased. In fact, these materials cannot withstand use in a solution containing 65% HNO.sub.3 which is known as a Huey test solution used in one of the common testing methods for testing the resistance of material to intergranular corrosion.
Therefore, under severe influence of nitric acid which enhances intergranular corrosion, it has been common to use stainless steels of extreme low carbon content such as JIS SUS 310 SLC and high-chromium, high-nickel austenite stainless steel, e.g., Uranus 65 (brand name of product from Creusot-Loire Co.) (C 0.03%, Cr 25.5%, Ni 20%, Nb 0.25%) forming a strong passive film stabilized by addition of elements such as Nb.
The austenite stainless steels having such high Cr and Ni contents, however, suffer from the following problems.
Namely, these stainless steels have only a small amount of a solid solution of carbon, so that, when heated to 500.degree. to 900.degree. C. or when subjected to heat produced by welding, the chromium carbides are preferentially precipitated in the grain boundaries enhancing the tendency of intergranular corrosion. In addition, these materials have high sensitivity to solidification cracking which makes the weld unreliable.
In contrast, ferrite-austenite two-phase stainless steels usually have high Cr contents and exhibit greater resistance to solidification cracking in welding. However, these stainless steels tend to cause a selective corrosion between structures due to, for example, the effect of weld heat. This tendency of corrosion is serious particularly in an environment having high concentrations of nitric acid. Thus, conventional two-phase stainless steel could not be used with sufficient reliability as nitric acid resistant material having welded structure.
To obviate these problems, the present inventors have proposed, as a metallic material having a higher resistance to intergranular corrosion than the conventional high-chromium, high-nickel steels in environments having high concentrations of nitric acid, a ferrite-austenite two-phase stainless steel of a 27Cr-8Ni-0.1N system. More specifically, this steel essentially consists of not more than 0.03% of C, not more than 2.0% of Si, not more than 2.0% of Mn, 25 to 35% of Cr, 6 to 15% of Ni, not more than 0.35% of N, and the balance Fe and inevitable impurities, and has an Ni balance value given by the following formula ranging between -13 and -9: EQU Ni balance value=Ni%+0.5.times.Mn%+30.times.(C+N)%-1.1(Cr%+1.5.times.Si%)+8.2
Unfortunately, however, no welding material has been developed which would suitably be used together with the above-mentioned two-phase stainless steel. Thus, there still is a demand for a welding material which can be used suitably in combination with the stainless steel of the type mentioned above to produce a ferrite-austenite two-phase weld structure thereby obviating the problem concerning corrosion.
In the past, welding material of a 25Cr--5Ni--2Mo system, as well as materials obtained by adding Cu, and the like additives to this system, e.g., WEL 25-5 and WEL 25-5 Cu sold by Nippon Welding Rod Company, have been used as the welding material which can produce a ferrite-austenite two-phase weld structure when used in combination with two-phase stainless steel of a ferrite-austenite system, for example SUS 329J1.
Also known are the welding materials such as E312 (C.ltoreq.0.08%, Si.ltoreq.0.9%, Mn 0.5-2.5%, Cr 17-20%, Ni 11-14%, Mo 2-25%, Cu.ltoreq.0.5%) and ER312 (C.ltoreq.0.15%, Si 0.3-0.65%, Mn 1-2.5%, Cr 18-32%, Ni 8-10.5%, Mo.ltoreq.0.5%, Cu.ltoreq.0.5%) as specified by the Standard of American Welding Society, as the welding materials suitable for use in combination with stainless steels of a Fe--Cr--Ni system.
These known welding materials, however, are not suitable for use in combination with the two-phase stainless steels which are required to withstand high concentrations of nitric acid, because they cannot produce sufficient resistance to corrosion due to the following reasons. Namely, these welding materials cannot offer superior resistance to corrosion by nitric acid, partly because they contain Mo and Cu which impair the resistance to corrosion in nitric acid environments and partly because the Cr content is as small as less than 25%. In addition, the small C content which usually falls around 0.08% decreases the resistance to intergranular corrosion in the weld part and makes these materials unsuitable for use in environments containing nitric acid which act to enhance intergranular corrosion.